Embodiments disclosed herein relate generally to an apparatus and method of flow management and CO2-recovery from a CO2 containing hydrocarbon flow stream.
Hydraulic fracturing, commonly known as hydro fracturing, or simply fracturing, is a technique used to release petroleum, natural gas or other substances for extraction from underground reservoir rock formations. A wellbore is drilled into the reservoir rock formation, and a treatment fluid is pumped which causes fractures and allows for the release of trapped substances produced from these subterranean natural reservoirs. Current wellhead fracturing systems utilize a process wherein a slurry of fracturing fluid and proppant (e.g. sand) is created and then pumped into the well at high pressure. When water-based fracturing fluids are used, a process referred to as hydro fracturing, the proppant, water and appropriate chemicals can be mixed at atmospheric pressure and then pumped up to a higher pressure for injection into the well. However, if fluids other than water (e.g. liquid CO2 or liquid propane) are used as the fracturing fluid, then these fluids must be kept at a sufficient pressure throughout the hydraulic fracturing system to avoid undesired vaporization. As a result, the blending of these fluids with proppant, chemicals, etc. must also be accomplished while the fluids are kept under a sufficiently high pressure.
CO2-fracturing employs CO2 to replace a significant portion, if not all of the water used in conventional hydrofracturing. The advantage of using CO2 is improved hydrocarbon production through reduced damage to the formation and proppant pack. Additionally, the environmental problems associated with hydrofracturing, such as soil contamination due to top-side fluid spills and use of clean drinking water sources are greatly reduced. Yet several factors limit commercial application. Such factors include cost of CO2, availability of CO2, flaring of CO2-rich hydrocarbon gases and effective proppant transport to name a few. CO2 as a fracturing fluid must be injected at the well site as a supercritical liquid. Typically, CO2-fracturing operations provide that the CO2 utilized for well stimulation is delivered from an external source, stored on site and blended with proppant under pressure.
Wellpad operations after stimulation are typically characterized in two distinct periods: a flowback period and a production period. The flowback period typically lasts between 2 to 4 days. During this period, operations at the well-pad may entail different steps such as millout of the plugs that isolate the various stages in a horizontal well, cleanout of the well-bore of the sand or other solid material, installation of production tubing, etc. The flow from the well during this period is a mix of sand/water/oil/gas and “trash” from the milling out of the plugs. In addition, the flow exhibits high variability in flowrates and compositions including starts/stops as required, accomplishing various tasks in each of the steps.
Subsequent to the flowback period, the responsibility is transferred to the production crew. During the production period, the flow is typically only oil/water/gas with very small amounts of sand, if any, and stable flowrates relative to the flowback period. The pressures during the flowback period are also higher (between 1000 to 2000 psig) compared to those of the production period. More specifically, during the production period pressures are high, typically between 750-1000 psig, and gradually decline over time depending on the well configuration. The flowrates are also relatively stable during this production period as they are mainly dependent only on the well conditions.
After CO2-stimulation, the flowback from a well during the flowback period is characterized by highly variable flowrates and compositions that change significantly over a period of days, e.g. gas flowrates changing from 10-15 million standard cubic feet per day (MMSCFD) to <2MMSCFD while CO2-concentrations change from approximately 100% to 40% over a period of 2 to 4 days. Typically, the CO2-rich flowback during this phase of operations is vented or flared because of the difficulties in designing a process that can keep up with the high variability in the flowrates and gas compositions over a period of 2 of 4 days.
During the production period, which may be from several months to several years, the gas flow rate and CO2-concentration would depend on when the flowback period was terminated and the operating conditions (tubing diameter, pressure, choke strategy) employed. The amount of CO2 in the flowback during the production period from a recovery perspective may be significant only for the first 30 days or so. The gas flowrates during this period would depend on the reservoir characteristics, the CO2-stimulation conditions, the extent of flowback handling during the flowback period, and the flow conditions during the production period. For example, the gas flowrates may change from 2-5 MMSCFD to 1-2 MMSCFD while the CO2-concentrations may change from 70% to 5%. After that initial period, the CO2 present above the sales or pipeline specifications is a nuisance that requires clean up to meet specifications.
Accordingly, there is a need for an improved flow management and CO2-capture system that provides for optimal recovery of CO2 from a CO2 containing hydrocarbon flow stream, such as a post CO2-stimulation flowback, for reuse. Optimal recovery of the CO2 is sought at desired specifications in keeping with equipment costs, footprint occupied at the wellpad, ease of commission, use, decommission and emission compliance.